1. Field of the Invention
The present invention relates to a system for determining certain operating parameters of a three phase AC induction motor. More particularly, the present invention relates to a method and/or apparatus used with a motor controller which determines RMS current through and RMS motor terminal voltage across the stator windings of a motor by using electrical parameters already known to the controller and without sampling discrete currents or terminal voltages.
2. Description of the Art
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating currents therein.
Given the design of a specific AC induction motor, there is an ideal range of currents and voltages under which the motor will operate properly in steady state. Thus, if the voltage across the stator windings is too low, the motor will stall (i.e. the motor will stop rotating). If voltage across, or current through, the stator windings is too high, motor components may heat up and damage may result. Therefore, it is extremely important that both RMS current and RMS terminal voltage be monitored and if either the RMS terminal voltage or RMS current is outside the desired range, the current or voltage should be altered. Furthermore, if either. RMS terminal voltage or RMS current is well outside the ideal range, the motor should be turned off until it can be properly serviced.
Most utilities which supply power to industrial motors supply well balanced purely sinusoidal three phase currents and voltages which have equal amplitudes and periods and are out of phase by exactly 120.degree.. Because the supply currents and voltages are purely sinusoidal, it is easy to determine RMS current and voltage values in order to verify that the values are within the ideal range. However, for various reasons, systems have been developed to alter the sinusoidal current and voltage wave forms at the point of utilization.
For example, in order to start an induction motor, current many times that of the steady state current is necessary as the stationary rotor is forced into rotation. In order to control the high start-up current needed to begin rotor movement, large alternating current electric motors are often operated by a controller. Many such controllers employ separate solid state switches connecting each stator winding to one of the three supply lines. Each solid state switch is formed by either a triac or a pair of back-to-back connected silicone controlled rectifiers (SCR's), commonly referred to as a thyristor.
The thyristor based control systems operate by introducing a non-conducting period, or notch, into every half cycle of a supply line voltage. By altering the duration of the notch, the motor terminal voltages can be limited and hence the current through the stator windings can be controlled. Thus, when the motor is to be started, an equipment operator applies a starting signal to the motor controller. The motor controller then gradually increases the amount of current applied to the motor by regulating the duty cycles of the thyristors coupling each phase of electricity to the motor. In doing so, the controller turns on each thyristor initially for only a portion of each half-cycle of the A.C. voltage for the corresponding electricity phase. The controller then gradually increases the half-cycle on time of the thyristors, thus gradually increasing stator currents, until the motor is at substantially full speed. This technique reduces the current consumption and torque of the motor during start-up as compared to a hard switching of the full supply line voltage across the motor.
In a similar fashion, the thyristor based systems are used to alter current levels when motor load is changed, to control current balance, motor torque, and slippage.
While thyristor based control systems have enabled precise control of stator currents and voltages, because they operate by introducing a non-conducting period into the purely sinusoidal supply voltage and current waveforms, they make it extremely difficult to ascertain RMS current and RMS terminal voltage values.
Referring to FIGS. 2(a) and 2(b), typical stator current 31 and terminal voltage 29 waveforms can be observed. As neither of these waveforms is purely sinusoidal, RMS current and voltage determination is computationally complex.
Typically, RMS current has been determined by placing transformers on each of the voltage supply lines. Each transformer isolates a current signal from the supply voltage and steps the current down from the high value found in motors (e.g. 10-1,000 A) to a value convenient to handle for signal processing (e.g. 100 mA). Deriving true RMS current values has typically required sampling the current in each line (at least 12 samples per cycle are needed, and usually 50 or more for RMS accuracy of a few percent). Next, each sampled current value has typically been squared, the squares averaged to get a mean square value, and then the square root of the mean square calculated.
In a like fashion, determining RMS terminal voltage in a system which introduces a notch period into the supply line voltage is computationally complex. As with RMS current, sampling and computing must be done in order to determine RMS terminal voltage.
These solutions to the RMS current and RMS terminal voltage measurement problems require a large amount of computational time which limits the ability of the motor controller to monitor and control other motor parameters. One solution to this problem would be to employ a faster controller which could accommodate many more calculations in a short period. A faster controller could do the RMS calculations and, at the same time, monitor and control other motor operations. However, a faster controller would still need to sample discrete current and voltage values which might require hardware in addition to the controller and the usual controller sensors.
In addition, while a faster controller might be a partial solution for future systems, systems which are already installed could not take advantage of faster computing without being at least partially rebuilt.
Therefore, it would be valuable to have a method and/or an apparatus by which RMS current and RMS terminal voltage could be derived using information already monitored by the motor controller without costly and time consuming discrete sampling and with only a minimal number of calculations.